To increase engine operating efficiencies and reduce unwanted emissions, it is known to alter the timing of the opening and closing of inlet and/or exhaust valves for internal combustion engines depending upon the engine operating conditions. As is well known, the optimal valve opening and closing, relative to the position of the engine crankshaft, for an internal combustion engine is dependent upon the engine operating speed, and to a lesser extent, other factors such as the engine load.
Ideally, the timing with which the inlet valves are opened and closed with respect to the crankshaft position should be changed independently of the timing with which the exhaust valves are opened and closed with respect to the crankshaft position. This change in the relative timing between the inlet and exhaust valves is typically referred to as the valve timing phasing.
In engines wherein one camshaft operates the inlet valves and a second camshaft operates the exhaust valves, the valve timing is adjusted by altering the position of each camshaft with respect to the synchronous drive (typically a toothed belt or chain) driven by the crankshaft and which rotates the camshafts and a variety of technologies and methods for achieving this are well known to those of skill in the art.
Until recently, it has not been possible to alter the valve timing in engines which employ a single camshaft to operate both inlet and exhaust valves, such as SOHC engines or engines employing push rods. However, recent development of concentric phaser camshafts, such as those described in U.S. Pat. No. 5,664,462 to Amborn et al., published international patent application WO 2006/097767 to Methley et al. and/or the SCP camshafts developed and sold by Mechadyne International Limited, Park Farm Technology Centre, Kirtlington, Okfordshire, UK now allow the alteration of valve timing in such engines.
These concentric phaser camshafts comprise a dual-acting camshaft wherein one of the set of inlet valve actuating cam lobes or the set of exhaust valve actuating cam lobes are fixed to a tubular outer camshaft member, while the other of the sets of inlet valve actuating cam lobes or exhaust valve actuating cam lobes are fixed to an inner camshaft member, mounted inside the outer camshaft member, and which is capable of relative rotation thereto.
While such camshafts provide obvious advantages and benefits, their manufacture is complex and/or expensive to achieve. Generally, the inner camshaft member is inserted into the outer camshaft member and an alternating stack of exhaust and inlet actuating lobes is mounted to the assembly of the inner and outer camshaft members.
The lobes affixed to the inner member are typically mechanically affixed to the inner camshaft member by pins inserted through bores in the lobe, then through corresponding slots in the outer camshaft member and finally into a corresponding bore in the inner camshaft member. The lobes which are affixed to the outer camshaft member are typically affixed by an interference fit wherein the lobe is heated to expand it and the assembly of the inner and outer camshaft members is cooled, via liquid nitrogen or the like, to allow the lobe to be positioned onto the outer camshaft member. Once appropriately placed, the lobe cools and the camshaft assembly warms providing an interference fit between the outer camshaft member and the lobe to fix the lobe in place.
While this assembly technique has been employed to date, it is expensive and time consuming to achieve. Generally, the tolerances for the rotational positioning of the lobes are generally one-half degree, or less. While it is relatively easy to create the bores through the inner camshaft member and the bores through the cam lobes to be affixed to it to correctly rotationally position those lobes on the camshaft, it is much more difficult to correctly rotationally position the lobes on the outer camshaft member while the interference fit between them is established.